1. Field of the Invention
This invention relates to devices and methods for improving the delivery of aerosol spray in an electrohydrodynamic (EHD) sprayer.
2. Background
The use of electrohydrodynamic (EHD) apparatus to produce aerosols is well known. In typical EHD devices fluid delivery means deliver fluid to be aerosolized to a nozzle maintained at high electric potential. One type of nozzle used in EHD devices is a capillary tube that is capable of conducting electricity. An electric potential is placed on the capillary tube which charges the fluid contents such that as the fluid emerges from the tip or end of the capillary tube a so-called Taylor cone is formed. This cone shape results from a balance of the forces of electric charge on the fluid and the fluid""s own surface tension. Desirably, the charge on the fluid overcomes the surface tension and at the tip of the Taylor cone, a thin jet of fluid forms and subsequently and rapidly separates a short distance beyond the tip into an aerosol. Studies have shown that this aerosol (often described as a soft cloud) has a fairly uniform droplet size and a high velocity leaving the tip but that it quickly decelerates to a very low velocity a short distance beyond the tip.
EHD sprayers produce charged droplets at the tip of the nozzle. Depending on the use, these charged droplets can be neutralized (with a reference or discharge electrode in the sprayer device) or not. The typical applications for an EHD sprayer without reference or discharge electrodes would be a paint sprayer or insecticide sprayer. Charged droplets in these types of sprayers may be preferred since the aerosol would be attracted to and tightly adhered to the surface being coated. However, with EHD apparatus used to deliver therapeutic aerosols, it is preferred that the aerosol be completely electrically neutralized prior to inhalation by the user to permit the aerosol to reach the pulmonary areas where the particular therapeutic formulation is most effective. Other drug delivery applications may dictate a small residual charge on the aerosol to accomplish some particular therapy.
During operation of the EHD device, the charged aerosol may leave the nozzle tip with very low velocity, and in the absence of some other force may build up in the region around the spray tip. This may be undesirable because the space charge on the aerosol may disturb the electric field and inhibit further aerosolization.
Another problem results from fluid wicking up the outside of the nozzle from the tip and either accumulating and/or flowing back to the tip where it may disrupt the Taylor cone. These disruptions and any other disruptions of the Taylor cone may result in a large variation in the size and size distribution of the aerosol droplets which is particularly undesirable in pulmonary drug delivery. A vertical orientation the nozzle reduces problems associated with the fluid collecting on or wicking up the outside of the capillary tube and associated fluid delivery means but it doesn""t solve the problem.
When administering pharmaceuticals to a patient these limitations on orientation of the EHD apparatus result in either the patients having to tilt their head backwards or to lie on their back when the aerosol is delivered on axis with the nozzle. Alternatively, the EHD apparatus can deliver the aerosol vertically on axis with the nozzle and an elbow means can be used to change the direction of aerosol flow to deliver the aerosol more horizontally. With this change in direction of the aerosol, there often is an appreciable loss in the quantity of the aerosol. The loss in quantity is a result of the fluid impacting and depositing on the walls of the delivery device, particularly in the vicinity of the elbow, instead of reaching the patient.
Therefore, an EHD aerosol sprayer is needed wherein the Taylor cone can be stabilized to prevent disruption and wherein the aerosol may be swept away from the region near the spray tip and be delivered more efficiently to the device exit. Of particular need, is an EHD aerosol sprayer that can improve the mass transfer efficiency of the device and improve the aerosol droplet distribution.
In accordance with the needs and objectives, the invention is an EHD aerosol sprayer wherein the Taylor cone is stabilized to prevent disruption (often resulting in a narrower aerosol droplet size distribution) and wherein the aerosol is swept away from the region near the spray tip and delivered more efficiently to the device exit.
The EHD aerosol sprayer generally includes a spray nozzle having at least one spray tip near which an aerosolizable fluid exits the spray nozzle, forms a Taylor cone and is aerosolized by EHD spraying and a gas flow deflector for directing a gas past the spray tip to sweep the aerosol downstream and away from the spray tip to promote further aerosolization. Preferably the gas flow is laminar (typically slightly greater than aerosol velocity) to reduce turbulence near the Taylor cone. Typically, the gas deflector is such that it directs the gas along at least a portion of the spray nozzle and past the spray tip substantially completely around the spray nozzle. Typically, the Taylor cone is elongated away from the spray tip in a preferred direction and the gas flow deflector directs the gas past the spray tip substantially parallel to the preferred direction of the Taylor cone. Typically, the spray nozzle is elongated and the Taylor cone extends away from the spray tip parallel to the nozzle and the gas flow is therefore substantially parallel to the spray nozzle as it moves past the spray tip and the Taylor cone. The EHD sprayer may also include a discharge electrode for neutralizing the electric charge on the aerosol droplets and for creating a corona wind to both move the aerosol and create an induced gas flow past the spray tip. Multiple spray tips are typically utilized to deliver higher volumes of fluid.
The invention further includes a high mass transfer electrohydrodynamic aerosol sprayer comprising: a spray nozzle in fluid communication with a source of fluid to be aerosolized, the spray nozzle having at least one spray tip near which the fluid exits the spray nozzle, forms a Taylor cone and is aerosolized by electrohydrodynamic spraying; and a gas flow deflector for separating a gas flow into at least two portions, for directing a first portion of the gas past the spray tip to sweep at least a portion of the aerosol downstream from the spray tip and for further directing the second portion of the gas away from the spray tip but thereafter into contact with the first portion of the gas and the aerosol downstream of the spray tip.
Typically the spray nozzle has at least one spray tip near which the fluid exits the spray nozzle, forms a Taylor cone which is elongated away from the spray tip in a preferred direction and is aerosolized by electrohydrodynamic spraying and wherein the gas flow deflector is further capable of directing the first portion of the gas past the spray tip substantially parallel to the preferred direction of the Taylor cone. Preferably, the gas flow deflector directs the first portion of gas substantially completely around the spray nozzle and directs the second portion of gas substantially completely around the first portion of gas and the aerosol downstream of the spray tip. Generally, a plurality of spray nozzles and spray tips are utilized to produce additional quantities of aerosol.
In some cases, it is desirable if the gas flow deflector surrounds the spray tips and is designed such that the gas velocity is lower near the outlet end than near the inlet end. This may be accomplished where the cross sectional area available for the gas flow near the inlet of the deflector is less than the cross sectional area near the outlet end. It is also desirable in some cases where the volume and/or flow rate of the first portion is less than the volume and/or flow rate of the second portion. A particularly useful EHD sprayer employing the corona wind to provide induced airflow comprises a spray nozzle in fluid communication with a source of fluid and having at least one spray tip near which the fluid exits the spray nozzle, forms a Taylor cone and is aerosolized by EHD spraying along a path parallel to a selected aerosol spray direction; a discharge electrode for producing ions near an ionized site on the discharge electrode and a corona wind from the ionization site along a desired path and being oriented such that the corona wind causes a flow of gas past the spray tip and sweeping at least a portion of the aerosol away from the spray tip; a reference electrode located between the spray nozzle and the discharge electrode; a first voltage source maintaining the spray nozzle at a negative potential relative to the potential of the reference electrode; and a second voltage source maintaining the discharge electrode at a positive potential relative to the potential of the reference electrode. A preferred sprayer typically has the discharge electrode oriented such that the desired path makes an angle of less than 90 degrees to the selected aerosol spray direction. The device may also include a second reference electrode located such that the discharge electrode is located between the first reference electrode and the second reference and a third reference electrode located such that the spray nozzle is located between the first reference electrode and the third reference electrode. The reference electrodes typically are at potentials that are positive with respect to the spray nozzle and negative with respect to the discharge electrode.
The invention further includes a method of producing and delivering an aerosol to a desired site by aerosolizing a fluid from a spray tip by electrohydrodynamic spraying; separating a gas into at least two portions, directing a first portion of the gas past the spray tip to sweep at least a portion of the aerosol downstream from the spray tip, directing the second portion of the gas away from the spray tip, and contacting the second portion of gas with the first portion of the gas and the aerosol downstream of the spray tip and sweeping the aerosol to the desired site. The method typically includes aerosolizing the fluid from a Taylor cone which is elongated in a preferred direction away from the spray tip at one end of an elongated spray nozzle, and directing the first portion of the gas past the spray tip substantially parallel to the preferred direction of the Taylor cone. Generally, the method may be used with a plurality of spray sites to increase the mass of aerosol.
Preferably the method includes directing the first portion of gas substantially completely around the spray tip and directing the second portion of gas substantially completely around the first portion of gas and the aerosol downstream of the spray tip to provide a sheath of gas around the aerosol to protect it from deposit on the device surfaces.
The deflector may be designed such that the first portion of the gas directed past the spray tips is less volume than the second portion of gas directed away from the spray tips. The ratio of the first to the second portions may be used to control the aerosolization and the delivery of the aerosol. The velocity of the first portion of gas may also be controlled by the flow resistance. A deflector that has an increasing cross section in the downstream direction will have the effect of reducing the velocity of the first portion of gas. This may be desirable in some situations to further stabilize the Taylor cone. The method is particularly useful in a pulmonary delivery device where the aerosol is generated by EHD.